1. Field of the Invention
The invention relates to a projection lens for imaging a pattern arranged in an object plane onto an image plane employing electromagnetic radiation from the extreme-ultraviolet (EUV) spectral region.
2. Description of the Related Art
Projection lenses of that type are employed on projection exposure systems used for fabricating semiconductor devices and other types of microdevices and serve to project patterns on photomasks or reticles, which shall hereinafter be referred to using the generic terms “masks” or “reticles”, onto an object having a photosensitive coating at ultrahigh resolution.
In order to allow creating even finer structures, various approaches to improving the resolving power of projection lenses are being pursued. It is well known that resolving power may be improved by increasing the image-side numerical aperture (NA) of the projection lens. Another approach is employing shorter-wavelength electromagnetic radiation.
However, improving resolution by increasing numerical aperture has several disadvantages. The major disadvantage is that the attainable depth of focus (DOF) decreases with increasing numerical aperture, which is disadvantageous because, for example, a depth of focus of the order of at least one micrometer is desirable in view of the maximum-attainable planarity of the substrate to be structured and mechanical tolerances. Systems that operate at moderate numerical apertures and improve resolving power largely by employing short-wavelength electromagnetic radiation from the extreme-ultraviolet (EUV) spectral region have thus been developed. In the case of EUV-photolithography employing operating wavelengths of 13.4 nm, resolutions of the order of 0.1 μm at typical depths of focus of the order of 1 μm may theoretically be obtained for numerical apertures of NA=0.1.
It is well known that radiation from the extreme-ultraviolet spectral region cannot be focused using refractive optical elements, since radiation at the short wavelengths involved is absorbed by the known optical materials that are transparent at longer wavelengths. Mirror system that have several imaging, i.e., concave or convex, mirrors that have reflective coatings arranged between their object plane and image plane and define an optical axis of the projection lens are thus employed in EUV-photolithography. The reflective coatings employed are typically multilayer coatings having, for example, alternating layers of molybdenum and silicon.
A reflective lens for use in EUV-photolithography that has four mirrors, each of which has reflective coatings with uniformly thick layers, is disclosed in U.S. Pat. No. 5,973,826.
Another EUV-photolithographic system is shown in U.S. Pat. No. 5,153,898. That system has a maximum of five mirrors, at least one of which has an aspherical reflecting surface. Numerous combinations of materials for multilayer reflective coatings suitable for use in the EUV are stated. Their layers all have uniform thicknesses.
Although reflective coatings with uniform thicknesses are relatively simple to deposit, in the case of imaging systems where the angle of entry, or angle of incidence, of the radiation employed on those areas of the mirrors utilized varies, they usually generate high reflection losses, since the thicknesses of their layers are optimized for a specially selected angle of incidence, or a narrow range of angles of incidence, only. Another of their disadvantages is a nonuniform pupil irradiance that causes a telecentricity error, structurally dependent or field-dependent resolution limits (so-called “H-V-differences or “CD-variations”), and generally lead to a narrowing down of the processing window.
Reflective EUV-imaging systems that have mirrors that have graded reflective coatings that are characterized by the fact that they have a film-thickness gradient that is rotationally symmetric with respect to the optical axis of the entire system are also known (cf. U.S. Pat. No. 5,911,858). Employing graded reflective coatings allows achieving a more uniform distribution of the reflected intensity over a certain range of angles of incidence.
Photolithographic equipment, or steppers, employ two different methods for projecting a mask onto a substrate, namely, the “step-and-repeat” method and the “step-and-scan” method. In the case of the “step-and-repeat” method, large areas of the substrate are exposed in turn, using the entire pattern present on the reticle. The associated projection optics thus have an image field that is large enough to allow imaging the entire mask onto the substrate. The substrate is translated after each exposure and the exposure procedure repeated. In the case of the step-and-scan method that is preferred here, the pattern on the mask is scanned onto the substrate through a movable slit, where the mask and slit are synchronously translated in opposite directions at rates whose ratio equals the projection lens' magnification.